Since 1911, over 100 species of exotic organisms have been deliberately introduced for biological control of various pests on Guam. Of these, 27 species were released against seven lepidopterous pests including one butterfly and one skipper. Recently, concern has mounted about the negative effects of these introduced biocontrol agents on nontarget species, particularly endemic island species. Guam may have suffered extinction of as much as 20–25% of its butterfly fauna. To determine the extent of movement and possible impacts of biocontrol agents on nontarget butterflies, mortality factors affecting juvenile stages ofHypolimnas anomala(Wallace) andHypolimnas bolina(L.) were studied. Both species were attacked by the same parasitoids and predators, which included native, accidentally introduced, and deliberately introduced species. Parasitoids killed 2.4% of the eggs ofH. anomala, and ants removed about 25% of the eggs. Parasitization was about 40% inH. bolina, and ants killed about 35% of the eggs. One species introduced for biological control,Trichogramma chilonisIshii, was found parasitizing the eggs, but it caused little mortality. No larval parasites were reared for either species.H. anomalasuffered heavy mortality from a disease. Both species were attacked by ants and two exotic, self-introduced predators. The major pupal parasite wasBrachymeria lasus(Walker), a deliberate biological control introduction.B. lasusparasitized 24.6% of the pupae ofH. bolinaand 2.9% of those ofH. anomala. Ants attacked 17% ofH. anomalapupae and 7% of those ofH. bolina. B. lasus and T. chilonisare polyphagous parasitoids that have the potential to affect a variety of nontarget species. This type of parasitoid, and highly polyphagous predators like ants, should require more intensive evaluation and review for use in classical biological control programs than more specialized species.